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Guide to the Application of Genotyping to Tuberculosis Prevention and Control

CDC Tuberculosis Genotyping Laboratory Procedures

Suspected False-Positive Cultures

If a submitting laboratory suspects that an isolate represents a false-positive culture, they should indicate this suspicion on the TB Genotyping Isolate Submission Form by entering “Yes” in the column labeled “Suspected False+.” If the submitting laboratory identifies a possible source for the false-positive culture (which could be one of the isolates in the current shipment, a previously submitted isolate, or a laboratory control strain), this information should be listed in the “Comments” field of the suspected false-positive isolate. In some instances, there may be a set of isolates that are suspect. All of these should be indicated by a “Yes” in the column labeled “Suspected False+” on the submission spreadsheet. These isolates will be genotyped in the usual manner, matched against all isolates from that program, and assigned cluster designations.

The genotyping laboratory will report the results to the TB program as usual. In addition, the genotyping laboratory will report the spoligotype and MIRU type and the results of the comparison with possible source isolates to the submitting laboratory as “the suspect isolate does (or does not) match the possible source isolate(s).” If there is a match, the cluster designation will also be reported. The genotyping laboratory will not report the result as confirming a false-positive culture. The submitting laboratory, clinician, and TB program must make that determination after considering all available information. A standard disclaimer will be included stating that genotyping methods are research procedures.

Because previous episodes of false-positive cultures resulted from cross-contamination by the H37Rv/Ra M. tuberculosis control strain, the genotyping laboratory will follow special procedures to ensure that cross-contamination by this strain is identified automatically. The PCR type for the H37Rv/Ra strain will be included by the genotyping laboratory in the file for each program and will be assigned the cluster designation “H37.” If a patient isolate is, in fact, the result of cross-contamination with this control strain, the genotyping process will assign it the H37 cluster number automatically. This is the only instance when the genotyping laboratory will report an isolate as being the result of a possible false-positive culture.